1. Field of the Invention
The present invention relates generally to automatic flight control systems for aircraft and more particularly to altitude acquisition and control portions of such flight control systems permitting a previously selected altitude to be quickly captured without enduring excessive accelerations of the craft normal to the flight path, even though the human pilot may have neglected to arm the automatic capture mode until after the normal automatic capture initiation situation has passed.
2. Description of the Prior Art
Automatic altitude control systems often include an altitude hold sub-system whose operation may be initiated by the human pilot. When the altitude hold mode is selected, the output of a barometric, aneroid, or other altitude pressure sensor is, in effect, clamped at the pressure altitude existent when the altitude hold mode was initiated. Then, a signal proportional to any deviation from the clamped altitude pressure is continuously supplied through the automatic pilot pitch channel, operating the pitch axis control surfaces so as to reduce the altitude pressure deviation to zero, thus maintaining the aircraft in level flight at the selected or clamped altitude.
Most altitude control systems include subsystems whereby a desired altitude may be preselected by the human pilot while the craft is still flying at an entirely different altitude. The altitude preselect sub-assembly is coupled with the altitude pressure sensor in such a manner that, when the craft reaches the selected altitude, a deviation signal is provided to maintain the craft at the selected altitude. In altitude preselect systems of the latter type, capture of the preselected altitude may be automatically accomplished. For example, after the new altitude has been selected, the human pilot may manually establish a desired rate of climb or rate of descent to the selected altitude. This vertical speed may be automatically maintained through the flight control system by providing a control signal to the pitch control channel thereof proportional to desired vertical speed and, as the craft approaches the selected altitude, the human pilot manually arms the capture mode. The capture mode is designed to permit the aircraft automatically and smoothly to capture the selected altitude without excessive acceleration normal to the flight path or mode engage transients. For this purpose, a null detector is used in which the altitude error h.sub.e (the difference between the selected altitude h.sub.s and the actual altitude h.sub.a) is compared with the altitude rate or vertical speed h. When the result of the comparison is a null, the capture mode apparatus is automatically engaged and any subsequent error is supplied to the automatic pilot pitch channel which controls the craft in a manner such that is asymptotically approaches and thereafter maintains the selected altitude. A limit is usually placed on the error signal supplied to the pitch channel to limit the magnitude of the pitch command, thereby to limit normal acceleration forces imposed upon the aircraft during the capture maneuver.
In altitude preselect systems of the foregoing type, certain precautions have been taken in the past to prevent large mode engage transients and excessive normal accelerations. For example, in a heavy traffic terminal area, the capture mode apparatus may inadvertently not be armed by the pilot prior to the altitude at which the altitude error h.sub.e is not equal to the altitude rate factor so that h.sub.e - Kh is not substantially zero. In such a case, the human pilot must himself manually maneuver the craft to acquire the selected altitude. Typically, the region around the capture point at which h.sub.e - Kh = 0 may be termed a window in which a null detector of the altitude preselect system can generate an automatic capture signal. If that window has already been passed by the craft, the null detector cannot produce an output and the capture mode apparatus is inhibited. Generally representative of such prior art altitude control systems is that of the Miller U.S. Pat. No. 3,240,446 for a "Preselect Altitude Control System for Aircraft", issued Mar. 15, 1966 and assigned to Sperry Rand Corporation.
The foregoing problems impose operational limitations on the aircraft flight crew, particularly during operations in the terminal area where the aircraft is operating at reduced air speeds and where frequent altitude changes are often demanded from the terminal area ground controller. With many other communication and navigation demands also being made upon the flight crew, it is possible that the pilot may neglect to arm the automatic altitude capture mode apparatus prior to reaching its capture window. In that event, he must now manually capture the selected altitude, diverting his attention from other cockpit duties.